An electrosurgical system that is arranged to controllably ablate a tumour and/or measure information concerning the tumour and surrounding healthy tissue is known. Such a system may use two channels: a first channel to perform controlled tissue ablation, and a second channel to perform sensitive tissue state (dielectric) measurements. The general principles relating to the operation of such a system are disclosed in WO 2004/047659 and WO 2005/115235.